Moist fine-grained ores continuously accumulate in relatively large quantities in LD-converters and blast furnaces, being recovered predominantly in sludge form by means of dust extractors. Hitherto, these moist fine-grained ores have generally been dumped together with dust-form ores accumulating in dry form. However, dumps such as these are no longer allowed for reasons of pollution control, with the result that the iron and steel industry has been forced to find ways of suitably processing the fine-grained ores in question. Table 1 below shows some examples of the chemical composition of fine-grained ores of the type in question which accumulate in dust form in blast furnaces and converters.
TABLE 1 ______________________________________ Examples of blast furnace Examples of Constituents dust LD-converter dust (in % by weight) A B C D E F ______________________________________ P 0.10 0.28 0.08 0.08 0.08 0.103 S 1.95 1.89 0.81 0.23 0.1 0.065 Na.sub.2 O 0.12 0.13 0.11 0.25 0.22 0.16 K.sub.2 O 0.28 0.34 0.22 0.08 0.05 0.03 Pb 5.9 1.50 n.d. 0.73 0.44 n.d. Zn 15.5 4.30 3.74 1.81 2.36 0.55 C.sub.total 21.9 31.8 43.2 2.10 1.7 1.41 Fe.sub.total 21.2 31.6 24.2 51.1 51.8 69.23 Mn 0.25 0.56 -- 0.78 0.8 SiO.sub.2 4.40 5.40 5.94 0.86 0.9 1.82 Al.sub.2 O.sub.3 2.49 1.40 2.84 0.13 0.16 0.39 TiO.sub.2 0.18 0.07 0.03 0.04 MgO 0.81 0.5 0.21 0.23 Cl 0.124 0.10 0.107 0.085 CaO 3.6 3.7 14.4 13.5 ______________________________________
Various processes have already been developed with a view to processing and also utilising these fine-grained ores accumulating in dust or sludge form. In these processes, the moist fine-grained ores are first dried, the agglomerates formed during drying crushed, the crushed agglomerates pelletised in pelletising pans or drums, and then subjected to heat treatment in a revolving tubular furnace for the purpose of reduction. During reduction in the revolving tubular furnace, zinc-containing and/or lead containing impurities present are simultaneously converted into oxides, removed from the revolving tubular furnace together with the exhaust gases, and separately recovered. These zinc and lead oxides are often present in concentrations of from 18% to 45% of zinc (=22.4% to 56% of ZnO) and from about 6 to 16% of lead (=6.5% to 17.2% of PbO), so that both metals are worthwhile recovering.
One disadvantage attending these known processes lies in the fact that the fine-grained ore present in the form of a moist filter cake has to be prepared at considerable expense for subsequent green pelletising which, if it is to be favourably carried out, requires a relatively narrow particle band width (i.e., as uniform a particle size as possible). Another disadvantage lies in the fact that the green pellets are only of relatively low strength and are introduced directly, i.e., without preliminary drying and prehardening, into the revolving tubular furnace in which they are subjected to severe mechanical stressing and, for this reason, are destroyed again to a considerable extent. The resulting dust and the pellet fragments seriously impede further processing and have to be re-pelletised in sintering installations for further metallurgical processing.
Applicants themselves have already developed a process of the kind referred to above. In this case, the fine-grained ore is reduced and subsequently agglomerated using a revolving tubular furnace for the final reduction step and a suspension preheater heated with the exhaust gases of this revolving tubular furnace for the preliminary reduction step and with introduction of a solid or liquid reducing agent into the gas pipe leading from the revolving tubular furnace to the preheater, the ore reduced to completion subsequently being agglomerated in the presence of a binder in a separate stage.
In such process the revolving tubular furnace is used only for the final reduction of the ore pre-reduced in the suspension preheater, agglomeration being carried out in a separate stage outside the revolving tubular furnace. In this way, the revolving tubular furnace can be specifically designed for reduction of the ore and can be made correspondingly shorter, because in contrast to other known processes, the revolving tubular furnace has no agglomeration zone. Since final reduction and agglomeration of the ore are carried out in two separate stages or units, it is possible in this way to obtain optimal reduction and agglomeration results. However, these good results are only obtained with dry, fine-grained ore, whereas the processing of moist, fine-grained ore, of the type accumulating in dust form for example in LD-converters and blast furnaces, is not possible.
Accordingly, the object of the present invention is further to develop a process of the type mentioned above (and the apparatus in which it is carried out) in such a way that, without losing any of its advantages, moist fine-grained ore, of the type accumulating in dust form in LD-converters and blast furnaces, can also be reduced and agglomerated in a fairly economical manner.